The invention relates to the pipeline transportation of wax-containing oil and more particularly to a method for reducing the pipeline pressure necessary to facilitate starting and restarting the pipeline flow.
Certain oils, such as crude petroleum and shale oil, contain a sufficient concentration of wax so that below a certain temperature, referred to as the pour point temperature, the wax content of the oil causes it to become viscous and the oil may even gel if permitted to stand for sufficient period of time below its pour point. The pour point temperature of an oil may vary widely depending upon the nature of the oil, its wax content and composition, and other factors. Thus, the pour point will vary widely depending upon the oil and can range from temperatures as high as 90.degree. F to as low as 0.degree. F. At temperatures above the pour point, the oil can be transported by pipeline efficiently and economically. At temperatures below the pour point, however, the wax content of the oil can begin to congeal and raise the viscosity of the oil causing a high pressure loss through the line and requiring excessive pump output pressures to move the oil.
A related problem with high pour point oils is caused by the fact that when the oil is allowed to come to rest, such as would occur during a pipeline shutdown at temperatures near or below the pour point of the oil, the oil will have a tendency to form a gel-like material having a high yield strength. Depending upon the length of the column of oil to be moved, the size and number of pumps, the diameter of the pipeline, the temperature and the yield strength of the oil at that temperature, the energy requirements to restart the oil flow may exceed the maximum allowable operating pressure of the pipe line resulting in a line shutdown until the pour point temperature is exceeded or the gel otherwise broken.
Depending upon the geographical location of the pipeline, the season of the year, whether the pipeline is buried or laid upon the bottom of a body of water or exposed to the atmosphere, the temperature of the oil in the pipeline can fall below its pour point and begin to thicken or gel in the line, particularly if flow should be interrupted for any reason. Consequently, pour point additives have been developed for mixture with an oil in order to lower its pour point and thus render more economical the pumping of the oil even at temperatures below its pour point. In addition these additives are designed to permit restarting of the line in the event of a shutdown where the temperature is at or below the pour point of the oil being pumped. The prior art is replete with various additives designed to inhibit the viscosity increase of the oil and to effectively reduce its pour point. These additives, however, must be added in sufficiently high concentration to inhibit not only the viscosity increase of the flowing oil when temperatures below its pour point are encountered but also in sufficiently high concentrations to inhibit the formation of the high yield strength gel in the event of a pipeline shutdown at temperatures below the pour point of the oil.
These additives increase the expense of pipeline transport of oil, particularly during the winter months in northern climates. Consequently a need exists for a more economical method for the transportation of high pour point temperature oils in which the restarting of oil flow in the event of a pipeline shutdown can be accomplished even at temperatures at or below the pour point temperatures of the oil being pumped.